1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, comprising a heat source for fixing toner on a recording medium and a fan for ventilating the inside of a housing.
2. Description of the Related Art
In recent years, it has been desired that an image forming apparatus such as a copier is designed in consideration of saving energy, for example, the Energy Star. The Energy Star is a standard for low power consumption regulated by the Environmental Protection Agency (EPA) of the U.S.A. in the International Energy Star Program. For saving energy, it is particularly effective to cut down power consumption in a fixing unit of the image forming apparatus, and specifically the following measures are possible.
First of all, it is effective to use a material having a relatively low melting point as toner. In the case where toner having a low melting point is used, a low temperature may be set as a fixing temperature with the result that power consumption of the fixing unit is reduced. In general, the fixing unit has a fixing roller composed of a fixed shaft member incorporating a heater, and a rotating sleeve which covers the shaft member. By using a very thin sleeve a period of time for warming up can be reduced, and, as a result, reduction of the power consumption of the fixing unit can be achieved.
In an image forming apparatus of Japanese Unexamined Patent Publication JP-A 7-160178 (1995), a temperature sensor is provided in a fixing unit and a cooling fan is provided in the image forming apparatus itself, which cooling fan is controlled so as not to be actuated before the temperature of the temperature sensor rises up to a sufficiently high temperature, whereby the power consumption of the cooling fan is cut down.
In an image forming apparatus of Japanese Unexamined Patent Publication JP-A 7-28356 (1995), a temperature sensor is provided in a fixing roller of a fixing unit and an exhaust fan is provided in the image forming apparatus itself to exhaust hot air coming from the fixing unit, and in a waiting state, the temperature for the fixing unit is set to a lower temperature than usual and the exhaust fan is controlled so as to rotate at a lower speed than usual, for the purpose of reducing the power consumption of the exhaust fan.
Japanese Unexamined Patent Publication JP-A 61-133968 (1986) discloses an image forming apparatus comprising a fixing unit having a fixing roller incorporating a heater, a cleaning unit for cleaning toner residue on a photoconductor, a fixing thermistor for detecting the temperature of the fixing roller, and a cleaning thermistor for detecting the temperature of the cleaning unit. In this fixing unit, an output of the heater is controlled in accordance with a temperature detected by the fixing thermistor and a temperature detected by the cleaning thermistor, whereby the power consumption of the heater can be reduced.
However, in the case where toner having a low melting point is used, toner collected by the cleaning unit melts under the influence of heat from the fixing unit. Once the toner has melted, it solidifies in the state of agglomeration. Accordingly it is hard to convey it to reuse. Even when the toner is conveyed to reuse, it has an adverse effect of, for example, causing inconsistency in density of an image formed on a recording medium.
Moreover, in the case where a sleeve of the fixing roller is very thin, a recording medium deprives heat from the sleeve, with the result that the temperature of the fixing unit is lowered. When the fixing temperature is lowered, toner is not fixed sufficiently, with the result that the quality of images formed on the recording medium is degraded.
Further, in JP-A 7-160178 and JP-A 7-28356, the operations of the fans are controlled in accordance with the temperature of the fixing unit alone, so that it is impossible to control the temperatures of all components except the fixing unit. As a result, the quality of images formed on a recording medium may be degraded by some changes in characteristics of electronic components in the image forming apparatus, or some changes in physical status of the toner.
Furthermore, JP-A 61-133968 discloses that only the output of the heater is controlled. Accordingly it is difficult to control temperatures of components except a fixing unit. In the case where importance is attached to the temperature of the fixing unit, overheat occurs in the components except the fixing unit, especially, in the cleaning unit, and hence, the same as in the case of using toner having a low melting point, it is also difficult to reuse toner. Even when the toner can be reused, it degrades the quality of images. On the other hand, in the case where importance is attached to the temperature of the cleaning unit, the toner is not fixed sufficiently because of reduction of the fixing temperature, with the result that the quality of images is degraded.